Interaction of Gonadotropin-Releasing Hormone Agonist and Antagonist with Progesterone, Prolactin, or Human Chorionic Gonadotropin during Pregnancy in the Rat*

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Abstract
Two GnRH analogs, one agonist and one antagonist, have been administered to pregnant female rats with or without concomitant treatments with progesterone (Po), PRL, or hCG. During the first week of gestation, Po was consistently capable of totally reversing the deleterious effect of the agonist, while PRL and hCG were slightly less effective. During the second week of pregnancy, only Po and hCG were effective. When the antagonist was injected during either the first or second week after mating, both Po and hCG could prevent the abortifacient action of the analog, while PRL was without effect. These data suggest that the antigestational properties of gonadotropin-releasing hormone agonists are at least partially mediated through inhibition of PRL secretion, while those of gonadotropin-releasing hormone antagonists do not appear to be. Additionally, hCG as well as Po proved capable of counteracting the antigonadal effects of both classes of analogs.